Vulnerability record · CVE-2016-0736 · published 27 July 2017
CVE-2016-0736: Apache HTTP Server mod_session_crypto padding oracle flaw
Apache · Http Server
Apache HTTP Server 2.4.0 through 2.4.23 encrypted mod_session_crypto data and cookies using ciphers with CBC or ECB modes (AES256-CBC by default) and provided no authenticated encryption. This makes the encryption vulnerable to padding oracle attacks, particularly in CBC mode, allowing encrypted session data to be decrypted or manipulated.
Description
In Apache HTTP Server versions 2.4.0 to 2.4.23, mod_session_crypto was encrypting its data/cookie using the configured ciphers with possibly either CBC or ECB modes of operation (AES256-CBC by default), hence no selectable or builtin authenticated encryption. This made it vulnerable to padding oracle attacks, particularly with CBC.
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Automated analysis
high priorityCVSS 3.0 base score is 7.5 (HIGH) with network reachability and no authentication, and EPSS is high, though no KEV listing or confirmed exploitation is recorded.
What it is
Apache HTTP Server 2.4.0 through 2.4.23 encrypted mod_session_crypto data and cookies using ciphers with CBC or ECB modes (AES256-CBC by default) and provided no authenticated encryption. This makes the encryption vulnerable to padding oracle attacks, particularly in CBC mode, allowing encrypted session data to be decrypted or manipulated.
Impact
An attacker who can observe and repeatedly submit modified ciphertext can recover plaintext from encrypted session cookies or data and potentially forge valid encrypted values. The CVSS vector indicates a confidentiality-only impact with no integrity or availability effect.
Attack surface
Reachable over the network with no authentication and no user interaction (AV:N/AC:L/PR:N/UI:N). The attack targets the encrypted session cookie or data handled by mod_session_crypto, so it applies only where that module is enabled and configured.
Exploitation
Not listed in CISA KEV and no ransomware associations are recorded. EPSS is high (0.49024, 98.8th percentile), and references are vendor and third-party advisories with no exploit tags, so no confirmed in-the-wild exploitation is documented in this record.
What to do
- Upgrade Apache HTTP Server to a version after 2.4.23 that addresses CVE-2016-0736, or apply the vendor/distribution patch (Red Hat, Debian, Gentoo, Apple, NetApp advisories are referenced).
- If mod_session_crypto is not required, disable it to remove the exposure.
- Where the module is needed, avoid relying on its encryption for confidentiality of sensitive session data and treat session cookies as untrusted input.
- Apply the referenced distribution errata promptly on all affected hosts, including bundled or embedded Apache deployments.
- Review session handling to ensure sensitive data is not stored in client-side encrypted cookies protected only by this module.
Detection
- Search web and proxy logs for repeated requests with malformed or altered session cookies that generate decryption or session errors.
- Monitor for high volumes of requests to session-protected endpoints from a single source, consistent with padding oracle probing.
- Audit Apache configurations for mod_session_crypto usage and flag hosts still running 2.4.0 through 2.4.23.
- Correlate error responses tied to session decryption failures across the web tier.
This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.
Affected products
1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.
References
Track CVE-2016-0736 inside VULONE
Watch it alongside the ransomware crews, C2 infrastructure and forum chatter that reference it, query it through the API and pull it into your SIEM over TAXII.
Related vulnerabilities
Same products first, then exploited flaws of the same weakness class.
Source: NIST National Vulnerability Database (record CVE-2016-0736), CISA KEV, FIRST EPSS (scores of 2026-09-26). This page is refreshed as NVD updates the record.
